Terebratulacea (Brachiopoda), Triassic to Recent: A Study of the Brachidia (Loops)

Abstract

The narrowly rostrate brachiopods of the Terebratulacea have long been a challenging subject. The interior details of many have long remained unknown. The usual method of studying the interior is by serial grinding (sectioning). Some workers are content with the information revealed by the sections; others give visual reality to the sections by reconstructing the loop and cardinalia. Noting the unlikely results of many of these reconstructions, a better way of revealing the interior suggested itself, i.e., revealing the loop by excavation of the matrix. This method, when matrix is workable, makes possible more accurate measurement and depiction of the loop. Making statistics of parts of the loop to use in comparing these structures in different genera is described. Certain characters and proportions of parts of the loop used in classifying genera are explained. External characters of generic importance are noted for each genus. The systematics of 208 genera of short-looped brachiopods of which 70% have revealed their loops by excavation or by being silicified are considered along with revision of families and the making of additional categories. It is indicated, with evidence, that the type of the Loboidothyrididae (Loboidothyridacea of Makridin) of the Jurassic has a short loop without conspicuous terminal points (flanges) rather than a long, long-flanged loop. It is recommended that the family definition of the Loboidothyrididae be altered to suit the facts of the structure of its leading genus, and that the Loboidothyridacea be abandoned. The Lobothyrididae, supposedly with short-flanged loop, has a loop with fairly long terminal points, and is thus in need of revision. It is suggested that <i>Pseudodielasma</i> of the Permian is stucturally a possible ancestor of Terebratulidae with abbreviated terminal points. The origin of the Loboidothyrididae (now Tchegemithyrididae) is postulated to be from the Triassic family Angustithyrididae of the Dielasmatacea according to Dagis (1974). The Permian <i>Ectoposia</i> is a possible ancestor of the Angustithyrididae. Available collections were inadequate for the study of the loop development of any genera except living ones. Information on the development of a Mesozoic long-flanged loop is known from the work of Dagis (1968) on <i>Viligothyris</i>. All known genera of the Terebratulacea were studied either by original preparations of the loop or from the literature. Lack of adequate collections prevented the detailed study of the Pygopidae. Fifty-eight genera of Jurassic brachiopods were developed to show the loop, fifteen of them Buckman's hitherto poorly known genera. An additional four unplaced genera were developed. Thirty-six Cretaceous genera revealed the loop, and fourteen genera of Tertiary Terebratulaceans are known from excavated loops. Forty-seven genera of short-looped brachiopods are known from description and serial sections only and without reconstruction of the loop. The loops of seven genera are unknown. Illustrations of all prepared genera are presented, and in addition, some reconstructions from serial section in the literature. Diagrammatic drawings of important loops of dissected specimens are illustrated. It is concluded that preparation of the loop, when possible, is preferred and is more accurate and less time consuming than reconstruction from serial grinding (section).